Residential alarms (hereinbelow referred to as “alarming devices”) that emit an alarm upon detecting an anomaly such as a fire, gas leak or the like have become prevalent, and in recent years, there has been an increasing trend to perform monitoring for anomalies such as fires in every room by installing a plurality of alarming devices in a single residence (for example, refer to Patent Document 1).
In this way, when a plurality of alarming devices have been installed in a residence, in the case of a person being present in a separate room from the room in which an anomaly has occurred, there is the risk of the alarm sound not being audible to that person. For that reason, one has been proposed in which a linked alarm is possible by connecting alarming devices with wires, and so in the case of one alarming device having detected a fire and emitting an alarm, it is possible to transmit an alarm signal from that alarming device to the other alarming devices to cause them to sound simultaneously.
However, since wiring work is required in order to connect the alarming devices with wires, the problem arises of higher cost. The problem can be solved by adopting wireless alarming devices. Moreover, due to the reduced power consumption of wireless integrated circuits, even if placed in an operating state of being capable of always receiving a signal in order to be capable of receiving an alarm signal from another alarming device, a battery life that can withstand practical usage of over, for example, five years, is ensured. Therefore, the environment for making wireless alarming devices commercially viable is being put into place.
In such a wireless alarming device, since it is not known when a signal indicating an anomaly will be transmitted from another alarming device, it is necessary to put the reception circuit section in a standby operation state in order to be able to receive a signal at anytime. However, since the power consumption becomes large by doing so, the reception operation is made to be performed discontinuously at every predetermined reception cycle.
Since there is no longer a need to always put the reception circuit section in a standby operation state with such a discontinuous reception operation, the current consumption of a reception circuit section decreases, and even if it is a wireless type alarming device, it is possible to guarantee a battery life exceeding five years.
In this discontinuous reception method, carrier sensing is performed by operating the reception circuit section at 10 second intervals. In the case of there being a carrier, the reception operation is continued for a fixed time required for signal reception, and then put in sleep mode, while if there is no carrier, it immediately enters the sleep mode.
In discontinuous reception by this kind of carrier sensing, shortening of the carrier sensing time is effective for reduction of the current consumption, and by utilizing a high-speed PLL synthesizer or the like, a cut in the current consumption is achieved by shortening by around 1 millisecond the required time for carrier sensing.